A fuel cell system serves to electrochemically react fuel gas, such as hydrogen gas, and oxidant gas containing oxygen to one another to allow an electric energy to be taken out from electrodes disposed on both sides of an electrolyte membrane. Especially, a solid polymer fuel cell employing a solid electrolyte attracts public attention as an electric power supply for an electric vehicle because of a low operating temperature and ease of handling. That is, a fuel cell powered vehicle is an ultimate clean vehicle on which a hydrogen absorbing device, such as a high pressure hydrogen tank, a liquid hydrogen tank and a hydrogen absorbing amorphous alloy tank, is installed to allow hydrogen, supplied from the same, and air including oxygen to be delivered to the fuel cell to accomplish reaction for taking out the electric energy from the fuel cell to drive a motor connected to drive wheels with only water remaining as emission matter.
In the solid polymer fuel cell, when using air as oxidant gas, nitrogen passing from a cathode to an anode through the electrolyte membrane remains in an anode system to lower a hydrogen partial pressure, resulting in deterioration in an efficiency of the fuel cell. For this reason, if the nitrogen remains in the anode system to some extent, there is a need for fuel gas containing nitrogen to be purged to the outside of the system from a purge valve located at an outlet of the anode. Such operation is sometimes referred to as “anode-purging”. When this takes place, the gas discharged from the anode is called as anode off-gas that includes flammable gas containing hydrogen. It is undesirable for hydrogen to be directly discharged to the outside and, hence, the purged anode off-gas is combusted in a catalytic combustor, thereby enabling mixed gas containing nitrogen and steam to be exhausted to the outside of the system.
Among technologies for combusting anode off-gas, a typical one technology is known from Japanese Patent Provisional Publication No. 7-78626 (see Page 3 and FIG. 1). This technology takes the form of a structure that is comprised of a mixture which permits a portion of cathode off-gas and anode off-gas to be mixed in a fuel cell, a regulator unit that regulates the flow rate of combustion exhaust gas in an exhaust gas delivery line, depending upon the flow rate of feed gas in a fuel line, based on a predetermined relationship such that a mixture ratio in the mixer between anode off-gas and cathode off-gas lying at a value deviated from an explosion limit.